CPU

Speed gains depend in some measure on which clock-speed options Apple offers.

For example, with the current Mac Pro the 3.46 Ghz CPU is not offered; you can’t buy a 12-core faster than 2.93 GHz — a loss of 18% on clock speed versus what is possible. Nor is there a 6-core 3.46 GHz option currently.

Bays and ports

We shall have to see if there are new goodies, like more than 4 bays, or extra bays for 2.5" SATA drives (e.g., SSDs), etc.

Thunderbolt

Once the Mac Pro sees Thunderbolt, and new far-cheaper Thunderbolt chip sets are out (this fall), the variety of Thunderbolt peripherals will explode, and the choices will expand, and prices will drop.

Native SATA III 6 Gb/sec ports

This will be a boon for a 6G SSD, but has little real-world benefit for fast hard drives (except perhaps on artificial tests and very specialized scenarios).

USB 3.0

Good and useful for simple add-ons, more so over time. Not really a high performance expansion option, but a Very Good To Have and a worthy replacement for moribund Firewire 800.

PCI 3.0 slots

For higher performance of PCI cards. Since most cards don’t use the existing PCI 2.0 bandwidth fully, this is a “nice to have”. Still, there are some specialty cards that might benefit.

Maximum memory

Potential for more addressable memory.

The maximum installable memory depends on the number of slots.

The maximum accessible memory depends on Mac OS X.

Today in Mac OS X Lion, Apple handicaps the existing 4/6-core Mac Pro to 48GB and the 8/12-core Mac Pro to 96GB instead of 64GB / 128GB (the same Mac booted into Windows can use the full amount). This is an Apple BUG.

Faster memory controller

The new on-CPU memory controller allows higher processor efficiency, especially important for the new 8 or 16 core CPU chips (up from 6 or 12).

Faster memory

Apple could stick with 1333 MHz memory, or move to 1600 Mhz (20% higher bandwidth). In practice, the actual speed gains are generally minimal, as seen with the current Mac Pro using 1066 vs 1333 MHz memory.